The present invention relates to an inertia sensor suitable for general use in an apparatus which controls the position and posture of a body in motion by detecting acceleration and angular velocity and a method of fabricating the same and more particularly, to an inertia sensor fabricated through semiconductor fabrication process and a method of fabricating the same.
The inertia sensor, especially, an acceleration sensor and/or a rotational angular velocity sensor (gyroscope or yaw rate sensor) finds their wide demand for sensors necessary for vehicle stability control, airbag and navigation systems of a car and for prevention of unintentional movement of cameras and compact video cameras. Prior arts will be described hereunder by mainly taking an angular velocity sensor, for instance.
A variety of angular velocity sensors such as a rotary type gyroscope using a rotating sphere or top and an optical fiber gyroscope using optical fibers have hitherto been developed. The rotary type gyroscope and optical fiber gyroscope are on the one hand highly precise but on the other hand are apt to be increased in size.
Under the circumstances, with the aim of reducing the apparatus size, a vibratory gyroscope devoid of rotary body and being operative to vibrate or rock a mass body has been developed and many kinds of piezoelectric type vibratory gyroscope reduced in size by mounting a piezoelectric device on a triangle pole or cylinder have been produced. In the vibratory gyroscope, however, small parts required to be precisely assembled, facing difficulties in fabrication. Further, any of the aforementioned angular velocity sensors including the vibratory gyroscope using the triangle pole or cylinder consist of a great number of separate parts, making it difficult to make the sensor portion (sensing part) integral with the associated circuit section.
Recently, to solve these problems, study and development of a compact vibratory gyroscope has been made actively by using a micromachining technique to which the silicon semiconductor fabrication process technique is applied. Through this technique, sensors can be produced at low costs and are suitable for mass production. Further it is expected that the sensor portion and the peripheral circuit section can be incorporated in one chip. The aforementioned technical trend stands with other sensors such as acceleration sensors.
Especially for vibratory gyroscopes, various types of sensors have been proposed and discussed, but till now, less studies have been made on a sensor of simple structure (with a high sensitivity) giving importance to mass production adaptability.
The basic operational principle of the angular velocity sensor fabricated by using the micromachining technique will be described by way of example of the sensor disclosed in U.S. Pat. No. 5,349,855.
The basic principle of the angular velocity sensor is that when a mass body constantly vibrating or rotating, moves in a direction along with a first axis with an angular velocity having a rotation axis parallel to a second axis which is vertical to the first axis direction, the angular velocity can be found by detecting a Colioris"" force generated in a third axis direction vertical both to the first and second axes. The Colioris"" force can be known by measuring an amount of displacement of the mass body. Hereinafter, the vibrating or rotating mass body is termed as a vibratory body.
The angular velocity sensor exemplified above is constructed of a vibratory body, a support structure for supporting the vibratory body, a driver for applying drive force necessary to vibrate the vibratory body and a detector for detecting the displacement due to Colioris"" force. The vibratory body is spaced apart from a substrate by means of a support beam of suitable shape and is driven electrostatically by using comb teeth electrodes. The direction of vibration is parallel to the substrate. Under this condition, when the rotation is applied to the vibratory body with the rotation axis parallel to the substrate but is vertical to the vibration direction, the vibratory body is displaced by Colioris"" force in a direction vertical to the substrate. This displacement is detected as a change in electrostatic capacitance by using an electrode disposed at the bottom of the vibratory body and the substrate, thereby measuring the Colioris"" force.
Examples of another angular velocity sensor fabricated by using the micromachining technique in which a drive electrode and a displacement detecting electrode are provided on a plane parallel to a substrate and a vibratory body is allowed to move on the plane only are disclosed in, for example, JP-A-09-189557 and JP-A-09-119942.
Now, the following points must considered carefully.
The provision of any of the aforementioned angular velocity sensors fabricated by using the semiconductor fabrication process presupposes a so-called surface micromachining technique in which the steps of forming a film (or player) such as insulating film and a polysilicon film on a silicon wafer and patterning the film by etching are repeated. In this case, in order to separate various structures from the substrate, the surface micromachining technique further needs a process in which a layer (sacrificial layer) that is to be extinguished in a later step is formed in advance. Layers incorporating the structures is then superposed on the sacrificial layer and the sacrificial layer is removed by etching in a final step. As a result, the vibratory body taking the form of a thin film is so formed as to be slightly spaced apart from the silicon wafer, making it difficult to cause the vibratory body to vibrate sufficiently.
On the other hand, many examples of an angular velocity sensor have been known which are fabricated by using a bulk micromachining technique, according to which, in contrast to the surface micromachining technique, a silicon wafer per se is etched by means of a device capable of working the wafer at a high aspect ratio thereby producing a structure. For example, JP-A-7-120266, JP-A-5-240874 and The Institute of Electrical Engineers of Japan, E-department (T. IEE Japan); Vol. 118-E, No. 12, ""98 show the examples as above. In any of these examples, electromagnetic force is used for drive means and the sensor is comprised of a worked silicon wafer, a glass substrate and a permanent magnet. In this case, the vibratory body is so worked as to have a sufficiently large volume (mass) and therefore, vibration necessary for sensing can be caused with ease.
In the sensor fabricated by the surface micromachining technique, it is easy to corporate the sensor portion with the detection and signal processing circuits of the sensor in one chip concurrently.
The surface micromachining technique, however, includes the step of etching the sacrificial layer in the course of process and so, after the vibratory body and support means are separated from the substrate in this step, cleaning is carried out during which the vibratory body tends to affix to the substrate and the fine interdigital patterns tend to affix to each other. Therefore, in order to raise the yield, special contrivance such as a freezing dry method must be employed. Further, when considering the integration with the circuit section, a high-quality protective film devoid of defects such as pinholes is required to be prepared in advance of the etching step to prevent the circuit section from being etched. As will be seen from the above, the etching step is very laborious and time-consuming and unless being assisted by new contrivance, it cannot be suited for mass production.
Further, in the sensor fabricated by the surface micromachining technique, the distance between the substrate and the vibratory body corresponds to the thickness of the sacrificial layer and is narrow, approximately amounting up to several xcexcm. Consequently, when the sensor is operated in the atmosphere, a large viscous resistance due to air acts on the vibratory body, raising a technical problem when we try to increase the Q value of vibration.
In the angular velocity sensor described in the aforementioned U.S. Pat. No. 5,349,855, the vibratory body is displaced by Coriolis"" force in a direction vertical to the substrate and during the displacement in the direction vertical to the substrate, the vibratory body so moves as to push off air prevailing between the substrate and the vibratory body and as a result, it undergoes large dumping (squeezed film dumping).
On the other hand, in the examples described in the JP-A-09-189557 and JP-A-09-119942, the vibratory body is driven and its displacement is detected in a direction parallel to the substrate and the influence of dumping can be decreased as compared to the sensor of the aforementioned U.S. Pat. No. 5,349,855. But the gap below the vibratory body is still narrower in these examples than in the sensor fabricated by the bulk micromachining technique, with the result that the vibratory body undergoes viscous resistance which is larger by one figure to double figures or more and it is difficult for the sensor to have high sensitivity when the sensor is used in the atmosphere.
Turning to the bulk micromachining type sensor, the vibratory structure is produced by etching an about 400 to 600 xcexcm thick silicon wafer per se and consequently, the thickness of the wafer by itself equals the thickness of the structure and the vibratory mass can be larger by about double figures in this type of sensor than in the surface micromachining type sensor. Since the Colioris"" force increases in proportion to the mass as shown in equation (1), the sensitivity of the sensor can also be increased:
Fc=2 mVxcexa9xe2x80x83xe2x80x83(1) 
where Fc is Colioris"" force, m is vibratory mass, V is vibration velocity of vibratory and xcexa9 is angular velocity to be measured.
Further, the distance between the vibratory body and the substrate can be relatively large, amounting up to about 50 to 100 xcexcm, a large Q value can be obtained even in the air. This also contributes to an increase in sensor sensitivity. In addition, since the etching step of the sacrificial layer can be removed in the fabrication process, this type of sensor will be fabricated more easily than the surface micromachining type sensor.
The bulk micromachining type sensor has more advantages than the surface micromachining type sensor as described above but disadvantageously, depending on the sensor structure, the sensor portion and the circuit section cannot be formed concurrently on the same chip.
Structurally, in the angular velocity sensor described in the JP-A-7-120266, the vibratory body is displaced by Colioris"" force in a direction vertical to the substrate and by measuring a change in electrostatic capacitance between one electrode provided to the vibratory body and the other electrode provided to the glass substrate serving as the support member and having a recess, an amount of displacement of the vibratory body can be detected. With this construction, because of the provision of the electrode on the glass substrate, wiring is not allowed to be laid on the silicon wafer only, making it difficult to realize the sensor and circuit integrated type construction. Further, in order to wire to the electrode provided on the glass substrate, through-holes must be formed in the glass substrate, thus making the process complicated. Further, since the displacement detection direction for the vibratory body is vertical to the substrate, the viscous resistance acting on the vibratory body is more increased in this type of sensor than in the sensor in which the vibratory body is displaced in a direction parallel to the substrate. In addition, when rotational angular velocity about an axis vertical to the ground is measured as in the case of yaw rate detection in cars, the whole of the sensor must stand vertically to the ground during packaging, thus making packaging laborious and time-consuming and making irregularity in sensitivity liable to occur after packaging. Especially when a sensor for automobile application is considered, the degree of freedom of installation is limited and therefore, realization of simple and steady packaging is an important problem.
In the sensor described in the JP-A-5-240874, all components necessary for sensing, inclusive of electrodes and wiring lines, are formed on the silicon substrate but the vibratory body is vibrated by electromagnetic force in a direction vertical to the substrate, so that a coil part must be formed on the vibratory body representing the movable portion to make the process complicated. Vibration of the vibratory body vertical to the substrate decreases the sensor sensitivity, adversely affecting packaging as has been described in connection with the aforementioned JP-A-7-120266.
In many types of acceleration sensor fabricated by using the bulk micromachining technique, a displacement of the vibratory body in a direction vertical to the substrate is detected by measuring a change in electrostatic capacitance between one electrode provided to the vibratory body and the other electrode provided to the glass substrate and during fabrication, the aforementioned problem is encountered in formation of the electrode on the glass substrate as has been described previously.
On the other hand, the angular velocity sensor described in The Institute of Electrical Engineers of Japan, E-department (T. IEE Japan); Vol. 118-E, No. 12, ""98 eliminates the problems in the aforementioned two known literatures and structurally, the vibratory body is driven and its displacement is detected in a direction parallel to the substrate and the magnet provided below the glass substrate is utilized effectively. But, as shown in FIG. 17, the silicon wafer constituting a sensing layer A is etched from not only its surface but also its bottom and consequently, work for turning up the wafer in the course of process is needed and a device such as a front-back aligner for bringing the bottom mask and the surface mask in line with each other is required. Further, when the wafer bottom is worked earlier than the wafer surface which is applied with patterning for formation of a vibratory body 11 and support beams 12 and patterning of wiring 21, the surface is contaminated and damaged, leading to a possibility that the wafer surface cannot be worked finely to raise a possible problem that the yield is degraded from the standpoint of mass production. Further, there needs a space D for anisotropic etching work as shown in the Figure. In addition, to permit work from the bottom to proceed, the both surfaces of the silicon wafer must be polished, raising a problem that the cost is increased as compared to the standard wafer polished on one side only and used during typical semiconductor production.
An object of the present invention is to provide an inertia sensor having high sensor sensitivity and construction suitable for high mass production adaptability and a method of fabricating the same.
Another object of the invention is to provide an inexpensive inertia sensor of high mass production adaptability which is fabricated by using the bulk micromachining technique to increase the thickness of a vibratory body and improve the sensitivity.
According to one aspect of the invention, to accomplish the above objects, an inertia sensor comprises a sensing layer and an auxiliary layer affixed to the sensing layer, wherein the sensing layer has a movable member and detection electrodes (conductors) for detecting a displacement of the movable member, the movable member is so constructed as to be displaced in a direction parallel to a junction surface between the sensing layer and the auxiliary layer, and the auxiliary layer has an opening in line with the movable member.
According to an embodiment of the invention, a sensor comprises a sensing layer and an auxiliary layer affixed to the sensing layer through a junction surface, wherein the sensing layer has a movable member displaceable in a direction parallel to the junction surface and detection electrodes (conductors) for detecting a displacement and the auxiliary layer has an opening formed on the junction surface side and having an area larger than an area which the movable member has on a plane along the junction surface.
According to another aspect of the invention, a inertia sensor comprises a sensing layer made of silicon and affixed to a glass substrate, wherein the sensing layer has a movable member and detection electrodes (conductors) for detecting a displacement of the movable member, the movable member is so constructed as to be displaceable in a direction parallel to a junction surface between the sensing layer and the glass substrate, and an opening is formed in the glass substrate in line with the movable member.
According to still another aspect of the invention, a method of fabricating an inertia sensor having a sensing layer and an auxiliary layer affixed to the sensing layer through a junction surface, wherein the method comprises the steps of providing a displacement member displaceable in a direction parallel to the junction surface and detection electrodes (conductors) for detecting a displacement of the displaceable member to form the sensing layer, providing a plate-like member and forming an opening having a larger area than that of the displaceable member in the plate-like member to form the auxiliary layer, and affixing the sensing layer and the auxiliary layer to each other such that the opening of the auxiliary layer confronts the displacement member.
According to the invention, in the displacement sensor comprising the sensing layer and the auxiliary layer affixed thereto through a junction surface, the movable member displaceable in a direction parallel to the junction surface is provided to the sensing layer and the opening in line with the movable member is formed in the auxiliary layer, so that sensor sensitivity can be improved as the mass of the movable member constituting the sensing layer increases. Further, as compared to the sensor having the opening formed in the sensing layer, the possible danger of adversely affecting the sensing layer surface by contaminating and damaging it can be reduced and a highly reliable and inexpensive inertia sensor of high mass production adaptability can be provided.
Further, by constructing the sensor such that the movable member is displaceable in a direction parallel to the junction surface between the sensing layer and the auxiliary layer, it is not required to form the structure of wiring lines and electrodes on the auxiliary layer and all components necessary for sensing, inclusive of the electrodes and wiring lines, can be provided on the sensing layer. Through this, the sensing portion can be formed integrally with its detection circuit and signal processing circuit, thereby realizing a highly reliable sensor device of simplified construction and high mass production adaptability.
In short, according to the invention, in spite of the fact that the sensor is constructed through bulk micromachining process, the main function of the sensor can be so packaged as to be concentrated to the silicon surface and therefore, even when integration of the sensing portion and the associated circuit section is considered, the compatibility of easy handling of the surface type sensor with sensitivity of the bulk type sensor can be met.